I can't understand why "とは" is being used in the following sentences:
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the とは particles are used to mark a definition or explanation of something. The structure is
<thing you are going to explain/define> とは <description>
Let's take a look at those two sentences. I've bolded the English words that are translated from とは
So in the first sentence the speaker says,
I have a question. That question is "why is everyone wearing a mask?".
the とは marks that the speaker is about the explain/define what his/her question is.
The second sentence is the same structure. The sentence is explaining what 花粉症 (or hay fever) is.
Hay fever (is/can be defined as/is explained as) the allergy to plant polen.
This site does a good job of explaining the basic uses of とは if you want to learn a little more.